scrollOffset is measured from the bottom of the chat buffer, so when
the agent appends new lines while the user has scrolled up to read
history, the visible window slides down through the buffer and the
content the user was reading drifts off the top.
Track the previous chat line count and column width across redraws.
While the user is in free-scroll (scrollOffset > 0) and the terminal
hasn't been resized, bump scrollOffset by the chat-length delta so
the visible content stays pinned. Compensation is skipped on resize
(line counts aren't comparable across reflows) and when following
the tail (scrollOffset == 0), where new content should keep pushing
the viewport as before.
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